In conventional power tools, e.g., hand-held power drills and screw drivers, it is known to use an electric drive motor which is coupled with a planetary gear reduction system to transmit the rotation of the motor drive shaft to a rotary chuck. Typically, the electric motor is a self-contained unit and the drive shaft of the motor is provided with a pinion gear which forms part of the planetary gear reduction system. To assemble the drive motor and the planetary gear system, a mounting plate is bolted onto the end wall of the motor housing with the motor drive shaft and the pinion gear extending through a central opening in the mounting plate. The other components of the planetary gear reduction system, i.e., the carrier plate and the planetary gears are assembled in a gear housing which supports a rotary output shaft for connection to the rotary chuck. The motor housing with the attached mounting plate is aligned with the gear housing and the pinion gear on the motor drive shaft is inserted between the planetary gears within the gear housing. The mounting plate is secured by a set of screws to the gear housing to complete the assembly of the drive system. It is also known to use bayonet-type fasteners to secure the gear housing to the mounting plate.